Universal Serial Bus On-The-Go (USB OTG) is a specification that permits electronic devices having a USB port to act as a host to an accessory device that is connected directly or indirectly to, or otherwise in communication with the host's USB port. More particularly, according to USB OTG, a primary electronic device (referred to as an A device in the specification) may provide power to an accessory device (referred to as a B device in the specification) over a voltage bus (Vbus). The OTG specification defines a Session Request protocol which allows an accessory device (i.e. a B device) to request a primary electronic device (i.e. an A device) to turn on its Vbus to provide power to the accessory device. Power supplied by the A device may be used to charge an internal battery of the accessory or, if the accessory does not have an independent source of power such as a battery, to provide power to on-board circuitry and components of the secondary device or to allow higher-demand power modules (e.g. such as class D amplifiers in an audio accessory) in the accessory to operate.
OTG specifies certain power restrictions for electronic devices. More particularly, according to the OTG specification, an accessory electronic device can only consume a certain amount of power. For example, according to the OTG specification, a primary device must be able to supply a minimum of 8 mA on Vbus at 5.25V.
When an accessory device wishes to consume an amount of power that that exceeds the amount provided for in the OTG specification, primary electronic devices supplying power often disable the USB port to prevent any power from being consumed. That is, if the accessory device wishes to consume an amount of power that exceeds the OTG-specified threshold, the primary electronic device may simply disable the Vbus so that the accessory device receives no power from the primary electronic device. Consequently, the accessory device may be unable to function.
Thus, the power scheme provided by USB OTG may provide a poor user experience when a user wishes to use an accessory with a power requirement that exceeds the thresholds provided in the USB OTG specification.
Like reference numerals are used in the drawings to denote like elements and features.